Fruits have been preserved against microbial spoilage by drying to very low moisture levels since time immemorial. It is now known that microbial spoilage is a function of water activity rather than moisture content. Water activity is the ratio of the vapor pressure of water in a material to the vapor pressure of pure water at the same temperature. Microorganisms have limits on their ability to prevent the loss of water from their cells and, if the water activity of their environment is too low, the cells cannot regulate the water loss and either become dormant or die. For example, few bacteria can survive at water activities less than about 0.85, few yeasts can survive at activities less than about 0.80, and few molds can survive at activities less than about 0.71.
In addition to its role in microbial spoilage, the water activity of a fruit is important when the fruit is used in products such as breakfast cereals. The cereal flakes, commonly derived from corn, wheat, rice, or other grains, are generally very dry and crisp with moisture contents of less than about 5.0 weight percent and water activities of less than about 0.5. If a fruit having a water activity higher than that of the flakes is mixed with the flakes, there will be a transfer of water from the fruit to the flakes, causing the flakes to become soggy. Accordingly, to retard or eliminate microbial spoilage and to reduce or eliminate the flow of water to the cereal flakes, fruits having low water activities are used in breakfast cereals. To date, the primary means for obtaining a fruit with a low water activity is to dry it to a low moisture level.
The use of extremely dry fruit in breakfast cereals is not without disadvantages. Extremely dry fruit tends to be hard, shrivelled, poor-tasting, and otherwise undesirable organoleptically. A soft, firm, pleasant-tasting fruit having a higher moisture level, but still having a low water activity, would be very desirable for use in breakfast cereals and in other applications.
A number of processes have been disclosed for infusing fruit with sugar or other humectants by immersion in a concentrated aqueous solution. When the fruit's water activity exceeds that of the solution and the solution's humectant activity exceeds that of the fruit, there is diffusion of water from the fruit to the solution and diffusion of the humectant from the solution to the fruit until equilibrium is reached. An increase in humectant content lowers the water activity of the fruit by reason of the humectant's ability to "bind" water.
U.S. Pat. No. 4,775,545, issued Oct. 4, 1988, (Augustine et al.) reviews a number of publications relevant to this topic (the relevant portions of said patent being incorporated herein by reference).
For example, Augustine et al. discuss U.S. Pat. No. 4,256,772, issued Mar. 17, 1981, (Shanbhag) which discloses a process for preparing fruits having a moisture content of about 18 to 34 weight percent and a water activity of about 0.40 to 0.65 for use in breakfast cereals. The fruits are infused with certain solutes to attain a lower water activity relative to their moisture content. Suitable solutes are edible polyhydric alcohols such as glycerol, mannitol, sorbitol, and propylene glycol, and sugars such as dextrose and sucrose. Shanbhag states that the process may be started with a fruit having a moisture content much higher than 15 weight percent and then dehydrating the fruit prior to infusion to produce an infused fruit having the desired moisture content. Example V of Shanbhag describes the infusion of commercial dehydrated peach slices.
U.S. Pat. No. 4,390,550, issued June 28, 1983, (Kahn) discloses in Example 11 another process for infusing fruit with solutes to control its water activity at about 0.45 to 0.65. Kahn bathes the fruit in a fructose-containing corn syrup comprising about 70 to 80 percent sugar solids, about 40 to 90 percent of which is fructose.
U.S. Pat. No. 4,350,711, issued Sept. 12, 1982, (Kahn) also discusses a process for infusing fruit with solutes. Kahn bathes the fruit in a series of at least two aqueous sugar solutions, the sugar in each of which comprises about 35 to 100 weight percent fructose, to gradually increase the sugar content of the fruit to about 32 to 55 weight percent. Kahn states that the gradual increase in sugar content minimizes "osmotic shock" and results in a reduced loss of volume by the fruit. Kahn teaches that the "driving force for the infusion of the sugar solutes of the infusion bath into the fruit is the osmotic pressure of the system resulting from the fact that the sugar solids concentration of the bath is greater than the water soluble solids content of the fruit prior to infusion." Kahn at col. 3, lines 3 to 8. Kahn further states that, in general, prior to infusion, the fruit is de-stemmed, the core is removed, and the fruit is washed and/or bathed in a solution of salt or acid (e.g., ascorbic acid).
Another fruit infusion process is disclosed in U.S. Pat. No. 4,551,348, issued Nov. 5, 1985 (O'Mahoney) and in U.S. Pat. No. 4,626,434, issued Dec. 2, 1986 (O'Mahoney). O'Mahoney bathes the fruit in an aqueous sugar solution having about 30 to 84 weight percent solids, about 35 to 100 weight percent of which is fructose, which is stabilized against dilution by the addition of a concentrated sugar solution into the bath and the withdrawal of a substantially equal volume of diluted syrup. O'Mahoney states that this process enables fruit solute levels to be raised to the desired level before detrimental shrinkage of the fruit due to dehydration occurs. O'Mahoney also states that, in general, prior to infusion, the fruit is de-stemmed, the core is removed, and the fruit is washed (including bathing in aqueous salt of acid) and dried.
U.S. Pat. No. 4,542,033, issued Sept. 17, 1985, (Agarwala) discloses an infusion process especially adapted for fruits having a high pectin content, such as apples, pears, cherries, etc. The fruits are cooked in an aqueous sugar solution (at least 30% solids) at a pH of about 1.5 to 3.75 and then cooled below their gelling point to form a gel within and between the cells of the fruit. Fructose is disclosed as a suitable sugar.
U.S. Pat. No. 4,364,968, issued Dec. 21, 1982, (Waitman et al.) discloses a process for preparing a dried grape product comprising infusing a grape with a carbohydrate (e.g., glycerol, fructose, or high fructose corn syrup) and drying the infused grape to produce a raisin-like fruit. At col. 4, lines 61-68, Waitman et al. favorably compare their process to the process of drying a grape to form a raisin, then infusing the raisin and then again drying the infused raisin.
U.S. Pat. No. 4,103,035, issued July 25, 1978, (Fulger) discloses a process for treating raisins and other fruits to improve their softness retention under storage conditions. The first step of the process is to contact the fruit with a hot, weak acid and then wash with water. After washing, the moisture content of the fruit is about 22 weight percent. The second step of the process is to contact the fruit with an edible polyalcohol humectant. Preferred humectants are glycerol and sorbitol. Other humectants disclosed include inverted sugar syrup. The third step is to wash with water and dry to a moisture content of about 12 to 20 weight percent. Fulger states that the acid treatment prior to contact with the humectant improves the fruit's absorption of the humectant. The treated fruit has a humectant content of about 3 to 20 weight percent. Water activities of the treated fruit are not given.
U.S. Pat. No. 3,952,112, issued Apr. 20, 1976, (Fulger) discloses another process for treating raisins and other fruits to improve their softness retention under storage conditions. In place of the acid treatment step disclosed in his '035 patent, Fulger discloses a number of other procedures which improve the fruit's absorption of the humectant. In particular, Fulger teaches that the removal of air bubbles and/or fruit waxes by application of a vacuum and/or washing with a surfactant or alkali is beneficial. Fulger also teaches that absorption of humectants is improved by raising the moisture level of the fruit prior to contact with the humectant:
It has also been found that the initial moisture content of the fruit, prior to exposure to the humectant, affects the rate at which the humectant will be absorbed. In general, the higher the initial moisture content of the raisins, the faster will be the rate of absorption. It is believed that at the higher moisture levels, the moisture dissolves fruit sugars in the fruit and enables the humectant to penetrate the fruit more easily.
Fulger at col. 4, lines 24 to 31.
Bolin, H. R. et al., "Effect of Osmotic Agents and Concentrations on Fruit Quality", Journal of Food Science 48(1):202-205 (1983), describe infusion of fruits in sucrose solutions and in high fructose corn syrups. Bolin et al. found that the high fructose corn syrup absorbed further in the fruit than the sucrose and that fruits infused in high fructose corn syrup exhibited lower water activities than those infused in a sucrose solution.
U.S. Pat. No. 4,418,082, issued Nov. 29, 1983, (Kahn et al.) discloses the generation of fructose within whole fruit or segments of fruit by inoculation with enzyme. At col. 2, lines 16-40, and col. 4, lines 26-29, Kahn et al. also refer to the enzymatic isomerization of glucose to fructose, as well as "inversion" of sucrose to fructose and glucose.
In summary, a variety of processes for infusing fruit with fructose and other humectants have been disclosed. The primary goal of many of the processes has been to increase the rate at which the humectant is absorbed by the fruit or to reduce damage to the fruit upon drying and on infusion. Kahn et al. '082 discloses the use of enzymes, as discussed above, to convert fructose to glucose as well as generally increase the fructose content. Augustine et al. disclose that predrying raisins enables one to obtain a higher fructose:dextrose ratio than is obtained by infusion alone.